We have isolated an NIH3T3 fibroblast cell line which grows in defined media in the absence of protein growth factors. The cells appear to release soluble factors which are capable of inducing serum-free growth of normal NIH3T3 cells, as well as several normal human diploid fibroblasts. These spontaneously-selected 3T3 cell lines, which appear to arise at very low frequency in 3T3 cell lines, express a phenotype clearly distinguishable from that induced by the ets-I and ets-2 genes and the myb-ets fusion oncogene, suggesting that multiple mechanisms are available for abrogation of serum dependence in 3T3 fibroblasts. We have tested DNAs from human leukemias containing 4:11 and 8:21 translocations for transfectable sequences which can dominantly alter the morphology, tumorigenicity, or serum dependence of 3T3 fibroblasts. The majority of these DNAs contain no detectable activity in multiple assays, suggesting that these translocations do not generate dominant oncogenes detectable by DNA transfection. We have generated a series of CHO cell lines carrying single human chromosomes which contain drug-selectable markers. Such cell lines will be useful as donors of specific chromosomes to determine the presence of genes capable of suppressing the transformed phenotype of established human tumor cells.